Thanks for the tips. Will give this a go as soon as I get some free time.
How long do you give the primer and/or flat black to dry between steel wool/scotchbrite "buffs"? (which I guess are like light sandpaper buffs to give the next layer a surface it will adhere to?)
Edit: do you go for Automotive flat black also, for the same reason as the automotive primer, or standard spray paint at that point?
After the Week before heavy use, or after the last round of spray paint, is there a sealer also?
Sorry about all the questions, just trying to work through the steps (and learn from other's experience).
Last time I tried this, I was a complete beginner. Minimal sandpaper and one coat on a modified nerf gun base. Looked great (was going for a steampunk esthetic).
Edges started flaking within about 2-3 days of handling (only gave it a day or two of time to dry). Trying to figure out the "why" as much as the "what" if that makes sense.
I live in Colorado, and it is very dry, so paint dries faster here than other places. Basically it just needs to be dry, so you can sand it. If it's not totally dry, it will smear, wrinkle, or "bunch" it off. Luckily it's flat paint, so it dries quickly. For me, that's maybe 30 minutes. One thing I do, to keep it moving, is every day when I come home from work I do 1 coat (e.g. sand yesterday's coat, set the parts up on toothpicks stuck in a box, and give it another coat, come back tomorrow).
The scotchbrite/steel wool is actually doing the opposite, at least for me. The coats I'm putting on are fairly light... just enough to give an even wet to the surface. With the flat paint, this results in a dusting textured effect after it dries. The scotchbrite knocks down the dust to an even dull paint. Then I put the next coat on it. The goal here is to even out the finish, remove dirt/hair/runs/texture/etc., so that the next coat is even smoother and hides even more.
I did not use automotive flat black, no. I used the absolute cheapest flat black paint, that I happen to still have several cans left over, from another project (shooting targets, where the finish wasn't important at all, because it was going to get shot off). If I was going out to buy paint, yes, I would probably have bought better paint, if only on principle.
I didn't use a sealer either. You could, if you wanted to, but I typically ask "Is there a reason to, on this project?". With flat black paint on dark charcoal pirmer on black ABS 3D printer plastic, who would ever see it if it got scratched? They wouldn't, so I don't think in needs the extra protection. However, on my Ultramarine (in my avatar), it was blue paint on black ABS, so I used matte spray to give it an even sheen between the gold and blue and add protection.
As for your flaking problem, it could have had to do with preparation. You need to make sure all oils (e.g. mold release from the factory, fingerprints, etc.) are off before you try to prime/paint. Use rubbing alcohol, mineral spirits, or acetone (don't use acetone on ABS or polystyrene plastic, as it is a solvent/cement for it, and chemically melts it). Paints really don't like sticking to plastics, especially injection molded plastics where they are extremely smooth (mirror finish, essentially). So light sanding/scotchbriting will give it some scratches to mechanically adhere to. You don't need to turn it into an arduous 10 hour prep job, but even a few minutes quickly wiping it off and scotchbrighting all the shiny spots will work wonders. A primer is important, and probably still better than all-in-one-paints that claim to prime too, and they even make special plastic primers for especially difficult to paint plastics (e.g. polyethylene, polyolefins, etc.). (Then finally, which you've acknowledged already, is that the paint doesn't reach full strength and adhesion for several days).